Molecular mechanisms which regulate the intracellular bioavailability of metals such as lead and cadmium have been studied in both mammals and marine invertebrates. High affinity cytosolic lead-binding proteins of 63,000 and 11,5000 daltons from kidneys of rats have been partially purified by gel and anion exchange chromatography, electrophoresis, saturation and sucrose density gradient analysis. These molecules were found to exhibit dissociation constants (Kd) for lead of 10tothe-8 M but addition of cadmium and zinc displaced 40-80% of the bound lead indicating an alteration of binding Pb capacity by these metals. The biological role of metallothionein (MT) in mediating the intracellular bioavailability of cadmium in renal proximal tubule cells following zinc induction of MT or zinc deficiency showed marked decreases in non-MT bound cadmium from injected CdMT following prior zinc induction of renal MT and increases in non-MT bound cadmium under following zinc deficiency. The binding site of a low molecular cadmium binding protein (CdBP) from oysters which is similar in size to metallothionein (MT) but which contains 7.6% cysteine and binds only 1.2 g atoms Cd/mole protein was also characterized. Scatchard analysis of 109Cd binding to purified CdBP showed a single class of site(s) with an apparent dissociation constant (kd) of 10tothe-7 M for Cd. An SH:Cd ratio of 2:1 for CdBP instead of the 4:1 ratio reported for MT was determined by SH group titration. Circular dichroism studies of CdBP incubated in vitro with a 2-fold excess of Cd or Cu disclosed marked reduction in the positive 259 nm Cd-S bond peak but no changes in other portions of the spectrum. These studies suggest that the lower Cd-binding affinity of CdBP relative to MT stems from the presence of only 2 SH groups at the Cd binding site.